A. Field of the Invention
The invention relates generally to methods of coating lenses. More particularly, the invention relates to methods of applying polarized coatings to curved lenses.
B. Description of Related Art
Polarized lenses block light of certain polarization states. By blocking horizontally polarized light, a polarized lens reduces glare that would otherwise exist through a non-polarized lens, such as glare off water, roads, and other objects. As a result of the reduced glare, objects become more distinct and true colors more clear. There are currently several different known systems for polarizing lenses for use in eyewear.
1. Film-Based Polarizing Systems
Certain of today's current eyewear products are fabricated by casting polyvinylalcohol-iodine films into a thermoset lens or by insert injection molding of a laminated polarized film to a thermoplastic lens. From a business perspective, these technologies are rigid and usually specific to mass production rather than made-to-order prescription ophthalmic lenses. The final optical properties of the resulting lens are determined by the film and are not easily altered. Additionally, film-based lenses require a separate inventory of polarized products, which can lead to increased costs.
Film-based products suffer from certain performance/technology shortcomings. Although the films have very high polarization efficiencies, the performance of the resulting lens is highly dependent upon the precise placement of the film within the lens. For example, if the polarization axis is not placed within three (3) degrees of the optic axis of a progress lens, the product is not acceptable. Also, a film placed on a progressive lens can greatly limit the final thickness of a wearer's lens due to the film's thickness. Furthermore, the precursor film to the polarization film can have cosmetic impurities/non-uniformities due to the nature of dying the polarization film (also known in the art as stretch films). Such non-uniformity, which can be observed as streaking in the film's coloration, can be exacerbated by the casting process, during which a thermal or chemical attack of the film can lead to dye bleach or further color non-uniformity.
2. Other Polarizing Systems
Example of lenses that have been polarized using a coating rather than film are shown in U.S. Pat. Nos. 4,648,925; 4,683,153; 4,865,668; and 4,977,028. Performance of the methods disclosed in these patents involves rubbing or scratching the lens prior to deposition of the dye used to form the coating. Such a process, commercially, is “dirty” and not readily adaptable or necessarily compatible with all lens materials and curvatures. To orient a dye molecule in these processes, the substrate must be scratched to form grooves of appropriate dimensions, which will in turn create a molecular orientation of the applied die that is favorable to alignment. The overall performance (contrast ratio=40) of such polarized lenses is relatively low. The scratching is also likely to induce some haze in the final product.
U.S. Pat. No. 2,400,877, discloses treating a substrate in some manner to produce an orientation that will, in turn, properly orient the polarizable materials that are applied to the substrate to form a polarized coating. Rubbing the surface of the substrate is disclosed as the preferred means of creating the appropriate surface orientation, although static electrical and magnetic fields are also disclosed for the same purpose. This patent mentions “spraying, flowing, pouring [and] brushing” as means of applying the disclosed films of polarizing materials to a surface. Dip coating is disclosed as one example of the disclosed application methods. Much of the patent is directed to describing means of fixing the applied polarized material, such as by controlling the evaporation and/or solidification of the film after it has been applied. The patent states that “[a]nother object of [the] invention is to provide polarizing films on curved and intricate surfaces and to provide films in any of unlimited colors and color combinations.” The patent also recites treating “polarizing filters for optical work of various kinds including photography, binoculars, goggles, windshields, mirrors, etc. . . . [and] lenses corrected for chromatic aberration . . . .” The patent does not suggest coating a lens by shear flow with a flexible apparatus or otherwise coating a surface that is not first treated for orientation in some way. The patent also does not suggest utilizing shear flow alone in coating a surface with a polarizing liquid.
Two systems have recently been proposed to form polarized coatings on flat surfaces using shear. The Optiva systems disclosed in U.S. Pat. Nos. 5,739,296; 6,049,428; and 6,174,394 include a blend of three self-assembling lyotropic liquid crystal dyes that, upon application of shear, orient to form various colored polarizers. These patents mention the use of coating rods, slot-dye (extrusion) coating, coating by capillary forces, and other methods as ways of coating a flat surface with, for example, a polymeric film or glass sheets. Because the orientation of the molecules occurs during the coating process, no surface preparation steps, such as rubbing, are necessary. This reduces the need for a specific alignment layer or reduces the incompatibility of surfaces on which liquid crystalline materials are not likely to align during application. The processes in these patent are suited to web coating a continuous roll of thin, flat polymeric films. They are not suited to use on non-flat surfaces.
U.S. Pat. No. 6,245,399 discloses a liquid crystal guest-host system that is aligned by shear forces. In this patent, the dye is not directly aligned by the shear flow. Instead, the orientation of the guest dichroic (pleochroic) dye is controlled by the host lyotropic liquid crystal material, which is oriented by shear flow. This patent does not suggest any shear flow application for a non-planar surface.